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Organics


Digestate: Using the right AD Langage


Digestate has long played second fiddle to the energy outputs of anaerobic digestion plants. However, there are many working hard to ensure that digestate is seen as a valuable and useful soil improvement product in the agricultural sector. David Burrows reports on work being carried out at Langage Farm in Devon.


David Burrows Freelance writer


L


ANGAGE HAS been a working farmstead for 900 years, and was even mentioned in the Domesday Book. The Plymouth-based farm is a little different today. There’s a full-scale, £1.8m processing


plant churning out a range of dairy products, mostly famous perhaps is the clotted cream, all helping towards a turnover of £3.3m. Customers include Sainsbury’s, Tesco and Waitrose. However, with this scale and enterprise


came problems. As production increased, milk yield from the herd of Jersey and Guernsey cows diminished. The soil had become compacted, lacking structure and the grass roots were stunted. Artificial fertiliser made the problem worse. The solution was to build an anaerobic


digester to produce ‘digestate’, for the farm. The results have been “impressive”,


according to the farm, with the grass treated remaining in its juvenile state for longer, so it remains “sweet and juicy”. It’s exactly how the cows want it, which means they eat more, it’s more nutritious and the yield rises. As well as the 13,000 tonnes of bio-


fertiliser, the plant also produces 500kW of electricity and 750kwh of heat. It’s the digestate that, as a farm, Langage


seems to be most excited about. A project with Plymouth University is


looking at how to modify the composition of the “fertiliser” to increase nutrients such as potassium or carbon so it’s matched to the crop being grown. In other words, they are tailoring the quality and nature of what comes out of the anaerobic digestion (AD) plant to the customers’ needs.


Playing second fiddle It’s an intriguing concept, but one which will hopefully highlight the importance and advantages of a vital output from the AD process. Indeed, amid all the feed-in- tariffs, renewable obligation certificates and renewable heat incentives, digestate has long played second fiddle to the energy output from plants. Energy will long make up the lion’s share of any revenue from AD, but what is the current market for digestate? How much do we know about it? And are


more farmers than those at Langage using it? WSP Environment & Energy has examined


the market for digestate and found a range of digestate values from a £5 per tonne disposal cost, to a revenue of £10 per tonne. As Mark Richmond, principal waste consultant, explains, this can present a challenge when assessing the feasibility of an AD facility. “Investors are attracted to the energy


incentives available to AD, but consideration also needs to be given to digestate off-takes and revenue potential. For example, an AD facility processing 30,000 tonnes of food waste per annum could produce 14,000 cubic metres of digestate. At prevailing rates this could


6 February 2 2012


equate to an annual revenue of £140,000 or a cost of £70,000.” This £210,000 variation introduces risk and


uncertainty into a project, says Richmond, who believes that greater certainty in the market for digestate would give investors and funders more confidence in the technology, in turn helping to bring more capacity on-stream. Others agree. Charlotte Morton, the


Anaerobic Digestion and Biogas Association’s chief executive, says adding another revenue stream for plants would “help hugely especially as gate fees for taking waste are expected to fall over the coming years, making it increasingly important to realise revenue from digestate as a product”.


Barriers Dr Adam Read is global practice director (waste management and resource efficiency) at AEA. “As I understand it, the viability of AD plants is based upon three things: energy production revenues, waste gate fees and having a sustainable outlet for digestate. “I also understand from discussions with


project developers that, when asked to finance these projects, the banks are particularly nervous about a particular project having only short term outlets for digestate. Looking at it from a long term financial risk perspective, if an AD plant were to suddenly find that it had to pay for the disposal of digestate rather than receive an (albeit small) income then the project might fail,” says Dr Read.


Courtesy of WRAP In the government’s anaerobic digestion


strategy and action plan, one of the biggest barriers to increasing the energy from waste from AD was the need for markets for the digestate. The Waste and Resources Action Programme (WRAP) is about to start investigating the standards and expectations of digestate in the UK. The aim of the project is to research and publish current market expectations for digestate and compost-like output (CLO) so that producers have more definition of the quality requirements for the existing and emerging markets for its use. In 2009, all the digestate produced was


supplied to farms (though WRAP is looking at its use in landscape and regeneration). However, it’s a new product and the


agricultural sector hasn’t yet developed the confidence in digestate as a suitable substitute for artificial fertilisers. There are some habitual behaviours that need to be overcome, says Read, as well as those around transportation: most of the digestate is water which is why the likes of Langage are looking at solar driers to make a stackable, dried product. There are also some concerns from the


retail sector in terms of the application of a material to food producing land which is essentially derived from a waste. However, there are many working hard


to ensure that digestate is seen as a valuable and useful soil improvement product in the agricultural sector. WRAP’s digestate & compost in agriculture’ project, for instance,


has already delivered key data about the nutrient content of digestate and typical figures for nutrient availability. The typical nutrient content of a food-based digestate is worth over £120/ha based on current fertiliser prices, according to WRAP. Its analysis also shows that “an impressive” 80% of the total nitrogen in food-based digestate is present as readily available nitrogen. “This high level of availability means that digestate can be used as a direct replacement for ‘bagged’ nitrogen fertiliser,” explains a spokesman. He adds: “We are also halfway through the


research that will explain precisely how much of the nutrient is taken up by crops and when the best times to apply it are. This data will help refine the fertiliser replacement value of digestate and enable farmers to calculate with precision the nutrients supplied by digestate to their crops, enabling them to cut applications of bagged fertiliser with confidence.”


Advantages of digestate There are a growing number of AD plants that operate across the UK that have demonstrated the benefits digestate can bring to farmland as fertiliser, says BiogenGreenfinish CEO Richard Barker. With Bedfordia Farms, BiogenGreenfinch has over five years’ experience of spreading digestate to farmland, and has seen both the economic benefit (reducing the amount spent on chemical based fertilisers), the environmental benefit (reduced carbon impact), and enhancements in crop yields, he explains. “AD digestate is still a relatively unknown


and recent product in the market. However its benefits are catching on. Much digestate is specified as a waste product, but with the development of the PAS110 digestate standard. “To date only three plants have passed the


PAS110 quality standard for digestate, though many more are in the pipeline, we expect that it will give farmers more understanding and confidence in the benefits of utilising digestate. It is more complex to apply to farmland than, say, pelletised chemical fertilisers and so again will take some education and potentially investment with farmers,” says Barker. Those at Langage Farm are also trying to


spread the word. Langage’s approach is very much focused upon making sure that the digestate is of a very high quality and tailored to the customer. The farm has an open day coming up for local farmers to come and hear more about the digestate. John Deane, part of a knowledge transfer


partnership between the farm and Plymouth University admits it’s not easy moving farmers out of their comfort zones to try something new, but the couple of farms that have tried it in the local area are said to be very pleased. “One is raving about it,” says Deane.


“We’ve had some positive results so far and I’m looking forward to what will happen in the spring when we’ll be able to compare the digestate with synthetic fertilisers better. “Hopefully we‘ll have a bit of an ‘Oliver


An anaerobic digestion plant processing 30K tonnes of food waste p.a. could produce 14K cu m of digestate www. r e c y c l i n gwa s t ewo r l d . c o . u k


Twist’ situation with farmers coming back wanting more.”


RWW Recycling & WA S T E W O R L D


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